Effects of short-term dexamethasone administration on corticospinal excitability.

PURPOSE: The short-term administration of glucocorticoids increases maximal voluntary force in healthy humans, but the underlying mechanisms remain poorly understood. The present study investigated the glucocorticoid effects on spinal and corticospinal pathways and on electromechanical properties of the tibialis anterior muscle in response to nerve stimulation.

METHODS: Twelve healthy men participated in a single-blind randomized study to receive either dexamethasone (8 mg · d(-1), n = 8 subjects) or placebo (n = 4 subjects) for 7 d. Group Ia afferent and corticospinal pathways were assessed, respectively, by recording the amplitude of the Hoffmann (H) reflex and motor-evoked potential (MEP) by transcranial magnetic stimulation. The ankle dorsiflexor torque and EMG activity during a maximal voluntary contraction (MVC) and muscle twitch evoked by electrical stimulation were also assessed before and after the intervention.

RESULTS: The MVC torque (+14%) and the associated tibialis anterior EMG (+16%) increased after glucocorticoid treatment (P < 0.05), whereas muscle twitch parameters did not change (P > 0.05). The H-reflex amplitude did not change (P = 0.58), but the MEP threshold was significantly (P = 0.008) reduced after treatment. Moreover, the slope of the MEP input-output relation and the silent period/MEP ratio increased (P = 0.049) and decreased (P = 0.029), respectively, after treatment. The amount of change in MEP amplitude and MVC torque were positively associated (r(2) = 0.59) for the dexamethasone group.

CONCLUSION: This is the first study indicating that short-term glucocorticoid administration in healthy subjects increased corticospinal excitability that contributed to enhance MVC torque.